I didn’t say anything, but the post you’re originally referring to is not talking about max speeds either, but ranges. I.e. average speeds. In a hilly city you might well reach 30 or even 60kph downhill. But it’s then even less likely to hit that as an average, at some point you will have to go up that hill again.
Totally agree on changing the view of city planners though. Even if I’m lucky enough to live in a city where this already very much reality.